Method for fusing aramid/aramid fibers

ABSTRACT

A method for welding aramid fibers, wherein a) at least one area of an aramid fiber is treated with an ionic liquid so that the aramid is partially dissolved, b) the aramid fiber is contacted via the dissolved area with another aramid fiber area with pressure being applied to the contact area, and subsequently c) the partially dissolved area of the aramid is re-coagulated.

BACKGROUND

The present invention relates to a method for welding aramid/aramidfibers, welded aramid fibers as well as shaped articles made of weldedaramid fibers.

According to the US-American Federal Trade Commission and DIN 60001aramids (aromatic polyamides, polyaramids) are long-chain syntheticpolyamides in which at least 85% of the amide groups are attacheddirectly to two aromatic rings. Two aramid polymers may be linked viahydrogen bonds and e.g. have the following structural characteristics:

Aramid is mainly produced as fiber and less frequently as foil. Aramidfibers are golden-yellow organic synthetic fibers. The fibers weredeveloped in 1965 by Stephanie Kwolek at DuPont and brought to marketmaturity under the trade name Kevlar. Aramid fibers are characterized byvery high strength, high impact resistance, high elongation at break,good vibration absorption and resistance to acids and bases. Inaddition, they are highly heat and fire resistant. Aramid fibers do notmelt at high temperatures, but start to carbonize from approximately400° C.

Well-known trade names of aramid fibers are Nomex® and Kevlar® fromDuPont or Teijinconex®, Twaron® and Technora® from Teijin. There aremeta-aramids (Nomex® and Teijinconex®), para-aramids (Twaron® andKevlar®) and para-aramid copolymers (Technora®).

Para-aramids are also referred to as poly-p-phenylene terephthalamide(PPTA), meta-aramids as poly-m-phenylene isophthalamides (PMIA); furtherchemical structures and trade names are available under the CAS numbers308082-87-3, 308069-66-1, 308069-57-0, 308069-56-9, 89107-41-5,63428-84-2, and 24938-60-1.

The para-aramid copolymer Technora® is a copolyterephthalamide withparaphenylene diamine and 4,4′-diaminodiphenyl ether. It has e.g. thefollowing structural characteristics:

Production of aramids is in general based on halides of aromaticdicarboxylic acid and aromatic diamines, such as para-phenylene diamine(PPD) und terephthaloyl dichloride (TDC):

The polymer is formed by reacting the two monomers in a solvent. Apossible solvent is hexamethylphosphoric triamide, which should,however, be avoided because of its carcinogenic effects. This may beachieved, for example, by conducting the polymerization in a slurry ofexcessive CaCl₂ in the solvent N-methylpyrrolidone.

Processing of aramid into fibers is in general carried out fromsolutions because the approach of direct spinning from a polymersolution has proven to be impractical and because the melting point isusually far above the thermal decomposition point. In this context, ahigh polymer concentration in the spinning solution is advantageous forfilament production and may result in high orientations.

Concentrated sulfuric acid is a good solvent for aramids in highconcentrations and anisotropic in nature. Possible ways for producingaramid fibers via polymerization and the use of sulfuric acid as solventor for direct spinning are known (Aramidprozess, Melliand Textilberichte1982, by Blumberg/Hillermeier). The spinning process may be a standardclassical wet-spinning process. The use of an air gap between thespinneret and the spinning bath, as is e.g. known from acrylic spinning,has advantages. After drying, the yarn usually has high strength and ahigh elastic modulus. In a second process stage, the yarn may bestretched, e.g. at temperatures of 300-400° C. This further increasesthe modulus at the same strength and reduced elongation at break.

In addition, ionic liquids as solvents for aramids, e.g. duringsynthesis (e.g. WO 2011/004138 A1, WO 2009/101032 A1) as well as forspinning composite fibers (US 2011/250162 A1) have been described in thestate of the art. This is advantageous because polyaramids are solublein almost no other solvent, except for concentrated sulfuric acid(highly corrosive and carbonizing), hexamethylphosphoric triamide(strongly carcinogenic) or suspensions of calcium chloride inN-methylpyrrolidone (toxic, teratogenic, difficult to handle). Otherhigh-polar solvents such as DMSO and DMF can only swell, but notdissolve aramid polymers.

Similar to carbon fibers, the fibers have a negative thermal expansioncoefficient in the direction of the fiber, i.e. become shorter andthicker upon heating. Their specific strength and their elastic modulusare considerably lower than those of carbon fibers. In connection withthe positive coefficient of expansion of the matrix resin, componentswith high dimensional stability may be produced. Compared to carbonfiber-reinforced plastics, the compressive strength of aramid fibercomposite materials is considerably lower; impact resistance, on theother hand, substantially higher. Aramids have extremely high stabilityagainst heat, they easily endure temperatures above 370° C. withoutmelting and are highly resistant to heat.

There are two different modifications that differ in particular withregard to the elastic modulus: “low modulus” (LM) and “high modulus”(HM).

Typical characteristics of aramid fibers are summarized in the followingTable 1:

TABLE 1 Low High modulus modulus (LM) (HM) Density in g · cm⁻³ at 20° C.1.44 1.45 Filament diameter in μm ≈12 ≈12 Tensile strength in MPa (N ·mm⁻²) 2800 2900 Tensile modulus of elasticity in GPa 59 127 Elongationat break in % 4 1.9 Therm. expansion coefficient in 10⁻¹⁰ · K⁻¹ −2.3−4.1 Thermal conductivity in W · m⁻¹ · K⁻¹ 0.04 0.04 Decompositiontemperature in ° C. 550 550

The data in Table 1 are taken from “Suter Kunststoffe: Aramidfasern(Kevlar) http://www.swiss-composite.ch/pdf/I-Aramid.pdf.”

The high modulus fibers are mainly used for structural elementssubjected to impact and shock stress, low modulus fibers for a varietyof uses, e.g. in bullet-resistant vests.

Aramid foils are often calendered from fabrics, but also produceddirectly as thin foils. They are often used as insulating materials,e.g. in transformers of insulation class C for temperatures up to 220°C., as base material for flexible printed circuit boards and as windowmaterials for accelerators and detectors. Nomex® fibers are, forexample, used in the AgustaWestland AW101 helicopter as compositematerial for covering close to turbine outlets and as textile in hot airballoons for the scoop and the lowest part of the envelope.

The most well-known uses of aramid fibers, e.g. para-aramid fibers, arein the security field (splinter and bullet protective vests, protectivehelmets, armoring for vehicles, cut resistant gloves, heat protectiveclothing). Aramid fibers are also used as replacement for asbestos inbrake and clutch linings and as sealing and reinforcement material, e.g.for fiber optic cables or rubber materials (tires). Another field ofapplication of aramid papers additionally is electric insulation. Theproducts may be used as sliding cover, groove insulation and phaseinsulation in electric motors as well as layer insulation intransformers. Aramid fabrics are also used for roofings in the buildingindustry, e.g. for stadium roofings. Here, they form a base materialthat is covered with PVC or PTFE to provide a UV and weather resistant,partially transparent membrane. Because of their toughness and tensilestrength as well as their low mass, aramid fibers are also often usedfor sports equipment, e.g. accessory cords, cords for paragliders, sailsfor sailboats and surf boards, hockey sticks or tennis rackets. Certainbicycle tires provide protection from broken pieces of glass and similarobjects by means of aramid inlays. Foldable tires often comprise aKevlar yarn instead of a wire.

In addition, aramid fibers or composite materials based on aramid fibersmay be used in aerospace applications, e.g. airplane cabin floors andinteriors, landing gear doors, wings, wing boxes and control surfaces,for pressure cylinders (e.g. oxygen pressure cylinders), for enginenacelles and engine safety rings, in airplane tires, in rotor blades, incable harnesses and air fright containers. Almost all modern jet enginescomprise aramid fabrics in the engine cowling in order to contain debriswithin the engine in case of blade-off events. The new Boeing 787“Dreamliner”, for example, consists of 50% by weight and 80% by volumeof fiber-reinforced high-tech composite materials, a substantial part ofwhich is made of aramid-based sandwich honeycomb cores.

In manned and unmanned space flight, aramid fibers are mainly used forprotection against flying space debris. In seagoing vessels, aramid isused for partition walls and other interior structures.

Aramid fibers are also used as reinforcing material in so-called“textile-reinforced concrete.”

In the automotive field, aramid fibers are e.g. used in tires, brakehoses, as vehicle protection, in brake linings, drive belts, automatictransmissions, fuel hoses, air-suspension bellows, cooling systems,turbochargers and sealing.

In the field of petroleum and natural gas production, aramid fibers areused for reinforcing pipes and pipelines as wells as for the protectionof supply lines, cables, risers, ropes and belts.

In telecommunications, aramid fibers are used for high-end opticaltransmission of data—as a further development of fiber optic cables.

Meta-aramid fibers are used specifically for fire protection. Theybecame known in fire-resistant clothing (e.g. protective clothing forfirefighters, race car driver suits etc.).

Membranes of modern high-performance speakers also often comprise aramidfibers.

In addition, aramid fibers are used as invisible threads by magiciansunder the name of “invisible Kevlar thread.”

During handling and processing, its high moisture absorption and low UVresistance have to be taken into account. The originally golden-yellowfibers take on a bronze-brown color upon UV radiation (sunlight) andlose up to 75% of their strength. Depending on storage, the fibers canabsorb up to 7% of water. Fibers containing too much humidity may bedried. In aerospace applications, a water content below 3% is common.Special micro-toothed cutting tools are required for cutting aramidfibers. Mechanical processing of finished fiber composite components isalso possible with high-quality processing tools or by water-jetcutting. Fiber composite parts are usually produced with epoxy resins.Chemical coupling agents are not known.

The state of the art shows that polyaramid fibers and foils offer avariety of great technical advantages, such as high tear resistance andhigh thermal resistance, but also have some disadvantages, such as:

-   -   aramid fibers cannot be recycled (except for cases in which the        above problematic solvents are used);    -   there is no chemical coupling agent;    -   aramid fibers may be glued, but not be welded (thermal        decomposition point lower than melting point).

These disadvantages in handling aramid fibers or foils result in aproblem with regard to shaped articles made of multiple aramid units,i.e. the joints between aramid units cannot show the same materialproperties as aramid.

For example, aramids may be used as inlay in tension members of conveyorbelts or conveyor facilities due to their tear resistance. Since ringclosure of the belt has to occur via a mechanical or bonded connectionof the aramid units, there is a weak point with regard to tearresistance. In general, joining aramid units may be regarded asproblematic, so that aramid fibers are often integrated with othersupporting and connecting materials, which results in increased materialexpenditure and higher weight of shaped bodies, which leads to furtherdisadvantages, e.g. in aircraft manufacturing.

Arrangement of fibers in fabrics results in a good, mostly tear-proofconnection. Aramid fabrics have a bullet-resistant effect, however, donot provide stab protection because the blade of a knife may penetratebetween individual fibers.

SUMMARY

Surprisingly, a method was now found for welding aramid which allowsovercoming the described disadvantages of the state of the art. Themethod for welding aramid fibers is characterized in that

-   -   a) at least one area of an aramid fiber is treated with an ionic        liquid so that the aramid is partially dissolved,    -   b) the aramid fiber is contacted at the partially dissolved area        with another aramid fiber area preferably while pressure is        applied to the contact area, wherein preferably the other aramid        fiber area has also been partially dissolved according to step        a), and subsequently    -   c) the partially dissolved area of the aramid is re-coagulated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a setup for welding a first aramid fiber102 and a second aramid fiber 103 on an object slide 101, the fibersoverlapping in an area with a length L;

FIG. 2 shows a cross-section of a setup for manufacturing a cylindricalshaped article, wherein an aramid fabric 202 is arranged on a supportbody 201, which fabric is surrounded by a grid 203 and is fixed by meansof clips 204;

FIG. 3 shows a cross-section of a setup for manufacturing athree-dimensional shaped body, wherein an aramid fabric 302 is embeddedbetween a first support grid 301 and a second support grid 303 and thesetup is fixed by means of clams 304.

DETAILED DESCRIPTION

“Aramid” according to the present invention includes meta-aramid,para-aramid and para-aramid copolymers.

As used herein, the term “aramid fibers” is intended to also includearamid foils, aramid fabrics, aramid parts, aramid coatings, e.g.lacquers, in addition to aramid fibers. According to the inventivemethod several fibers may be welded together or single aramid fibers maybe welded with themselves. For example, the method may be used toconnect a single aramid fiber at its ends in order to provide an aramidfiber ring. Here, on end represents the partly dissolved area and theother end represents the other aramid fiber area. The aramid fiber areamay also extend over the entire aramid fiber or foil.

Ionic liquids are in the sense of acknowledged literature (e.g.Wasserscheid, Peter; Welton, Tom (Eds.); “Ionic Liquids in Synthesis,2^(nd) Edition,” Wiley-VCH 2008; ISBN 978-3-527-31239-9; Rogers, RobinD.; Seddon, Kenneth R. (Eds.); “Ionic Liquids—Industrial Applications toGreen Chemistry,” ACS Symposium Series 818, 2002; ISBN0841237891)—liquid organic salts or salt mixtures consisting of organiccations and organic or inorganic anions with melting points below 100°C. Additional inorganic salts may be dissolved in these salts, as wellas molecular adjuvants. For the purposes of the present application, weregard the melting point limit of ionic liquids arbitrarily set at 100°C. in a wider sense and also include salt melts having a melting pointabove 100° C., but below 200° C. Ionic liquids have very interestingproperties, such as in general very low to non-measurable vaporpressures, very wide liquidus ranges, good electric conductivity, andunusual solvation properties. These properties predestine it for the usein various fields of technical uses. For example, ionic liquids are alsoknown for welding natural fibers (Haverhals, L. M.; Foley, M. P.; Brown,E. K.; Fox, D. M.; De Long, H. C.; In Ionic Liquids; Science andApplications; 2012). As shown in Table 1 in, Haverhals et al., theproperties of aramids are in many aspects more similar to those of steelthan natural fibers. It was thus not to be expected that this use ofionic liquids was transferrable to aramids because these syntheticmaterials differ from natural fibers such as cellulose in theirstructure, constitution and dissolving behavior.

When using ionic liquids, optimizing the properties for the respectiveuse within wide limits can be achieved by varying the anion and cationstructures or their combination, which is why ionic liquids aregenerally called “designer solvents” (see, for example, Freemantle, M.;Chem. Eng. News, 78, 2000, 37).

Suitable ionic liquids for the inventive method are selected so that thearamid is partially dissolved. Here, “partially dissolving” means thatthe structure is not completely dissolved, but contact or interactionbetween individual aramid polymer chains is loosened or broken up.Dissolve partially, however, also means that a part of the structure ofthe aramid unit remains intact. For example, the core of an aramid fibermay remain unchanged, while the surface area is dissolved in the courseof the method. Gelling aramid fibers is also regarded as partiallydissolved according to the invention because the main shape and theunity as a fiber are maintained. Completely dissolving aramid fibers,i.e. dissolving them with the loss of the entire unit, is not comprisedin the term of dissolve partially.

The inventive method is based on the finding of the present inventionthat an appropriate selection of the cation, the anion and optionaladditives provides ionic liquids that are suitable to partially dissolvearamids according to the present invention. Such ionic liquids are alsoreferred to as “ionic liquid(s) according to the present invention”herein.

It is to be expected that the interaction forces between individualpolymer chains of a fiber are partially disrupted during these methodsbecause the ionic liquid penetrates between the chains. The physicalproximity of the fibers in step b) of the method allows polymer chainsof different fibers to get into contact with each other. Subsequently,the interaction between polymer chains may be restored, for example, byremoving the ionic liquid. Because of the arrangement created, thefibers coagulate so that the polymer chains of the originally separatedfibers start interacting. In the course of step c), the materialproperties of the aramids are at least partially restored and the fitand form closure desired for welding is achieved between the individualaramid fibers.

While steps a) and b) of the method may be carried out in the order a)followed by b) or in combination or in reversed order, step c) iseffectively carried out subsequently to the other steps. Steps a) and b)may, for example, be combined so that the aramid fibers are firstcontacted and form a fabric, which is then treated with the ionic liquidas a fabric. It may be advantageous to repeat individual steps.

Advantageously, the inventive method thus provides an uncomplicatedwelding method and allows “reshaping” of aramid fibers by partialdissolution, connection and re-precipitation.

Coagulation of the dissolved aramid may be carried out by addingappropriate anti-solvents as liquid- or vapor-phase or by absorption ofair humidity because in particular suitable ionic liquids may bestrongly hygroscopic. Alternatively, the ionic liquid may be removed byheating. Depending on the temperature during heating, there are twomechanisms for removing the ionic liquid: a) heating to above itsthermal decomposition point in order to convert it to gaseousdecomposition products, optionally under application of vacuum, or b)heating to below its thermal decomposition point, as long as the ionicliquid has a vapor pressure high enough to allow distilling it off,optionally in vacuum. “Reshaping” also comprises shapingthree-dimensional parts and structured fabrics, for example atwo-dimensional aramid fabric/textile may be sprayed with an ionicliquid and be pressed into a three-dimensional shape under heat and/oranti-solvents.

Ionic liquids according to the present invention have the generalformula[A]⁺ _(a)[B]^(a−)  I

wherein

a is a number of 1, 2 or 3,

[A]⁺ is an organic cation, and

[B]^(a−) is an anion.

In a compound of Formula I of the present invention, [B]^(a−) refers toany coordinating anion with a negative charge a that is in line with thenumber of existing cations a in [A]^(a+).

Here, a in [B]^(a−) refers to a number of 1, 2 or 3. An example ofdouble-negatively charged anions is carbonate. An example of atriple-negatively charged anion is phosphate.

As used in the present patent application, suitable ionic liquids referto “coordinating” ionic liquids. A “coordinating” ionic liquid, as usedin the present patent application, is one that contains coordinatinganions. Coordinating anions are characterized in that they comprise freeelectron pairs that are able to form coordinative bonds with electronpair acceptors (Lewis acids) or proton donors (Bronstedt acids). In thecase of proton donors, the protons are covalently bonded to an atom thatis electronegative towards the proton (e.g. O, N, F), so that the bondis polar (“acidic protons”), however, the proton is not cleaved off butforms a second, coordinative bond with the coordinating anion of theionic liquid (hydrogen bond). Particularly good coordinating anions areonly present when the free electron pairs sit on small heteroatoms withhigh charge densities. This is especially the case with anionscontaining oxygen atoms, in particular alkyl oxides, aryl oxides,hydroxide, or carboxylate, or with fluoride, chloride and bromide.

As used in the present document, the term “coordinating ionic liquid”comprises, in addition to coordinating ionic liquids, also mixtures ofdifferent coordinating ionic liquids as well as mixtures of coordinatingionic liquids with co-solvents or anti-solvents, so that e.g. inaddition to the solvents of groups II and I, as described in Example 3of the present application, non-coordinating ionic liquids may also beadded as co- or anti-solvents.

In addition, it was shown that it is preferred for cations to be presentas “quaternary” compounds. As used in the present application,“quaternary” means that all valences of a nitrogen or phosphorus atomare stably and organically bound so that no more lone, non-bindingelectron pairs exist. This applies to fourfold alkylated ammonium orphosphonium salts as well as to nitrogens integrated into aromatics(heteroaromatics) with a third substituent, which is not hydrogen, e.g.pyridinium salts. Decisive for suitability in the invention is theproperty of a constant positive charge. This property is also providedby protonated guanidinium because the hydrogen has such a low aciditythat a constant positive charge is also guaranteed here. For guanidine,a pK_(a) value in water of 13.6 to 13.7 is given. Substituted guanidinederivatives may be even more basic, with pK_(a) values above 14, whichmay be determined in non-aqueous media, or less basic, e.g. pK_(a)˜10for phenyl guanidine, via the electron acceptor residue. The inventionincludes protonated cations with a pK_(a) value ≥10, preferably ≥13. Itis a finding of the present application that ionic liquids withdeprotonable cations are not suitable for dissolving or partiallydissolving. In Example 2, the Comparative Examples No. 22-23 have acidichydrogens and support this finding.

It may also be deducted from Example 2 that the size of the cation hasan impact on whether an ionic liquid dissolves aramids completely orgels/partially dissolves them. This may be controlled, for example, viaa suitable side-chain length of the alkyl residues. While a1-butyl-3-methylimidazolium chloride in Example 3 completely dissolvesthe aramid fiber, it is only gelled by 1-decyl-3-methylimidazoliumchloride in Example 12. It is assumed that the smaller, flexible cationsenable the cation to penetrate between the individual polymer chains.The requirement of a cation to penetrate deeply might be explained bythe fact that in addition to hydrogen bonds, aromatic interactions mightalso play a role between the aramid chains. The presence of positivecharges between the chains may replace the purely aromatic interactionswith cation-aromatic interactions and thus lead to a completedissolution. Ionic liquids, the cations of which are not able topenetrate between the polymer chains that easily because they are biggeror bulkier, seem to promote gelling or partial dissolution. A personskilled in the art can appropriately choose the size of the cationsbased on the examples disclosed herein.

In order to identify suitable ionic liquids for dissolving or partiallydissolving, Kamlet-Taft solvent parameters may be used. The Kamlet-Taftsolvent parameters describe the hydrogen bond donor property (u), thehydrogen bond acceptor property (3) and the bipolarity/polarizability(π*) of the solvent (“Solvents and Solvent Effects in OrganicChemistry”; Christian Reichardt, WILEY-VCH, Weinheim 2003, ISBN3-527-30618-8; Kamlet, M. J.; Abboud, J. L.; Taft, R. W. J. Am. Chem.Soc. 1977, 99, 6027; Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976,98, 377; Kamlet, M. J.; Hall, T. N.; Boykin, J.; Taft, R. W. J. Org.Chem. 1979, 44, 2599; Taft, R. W.; Kamlet, M. J. J. Am. Chem. Soc. 1976,98, 2886). They may be measured by means of a so-called solvatochromicshift and are well-known to the person skilled in the art. It has beenshown that ionic liquids with high β values, i.e. with good propertiesas hydrogen bond acceptor, and low α values, i.e. poor capacity ashydrogen bond donor, are particularly well suited. The following tableshows several parameters from the literature, wherein multiple entriesresult from contradictory literature references:

TABLE 2 No. Ionic liquid ^(X) CAS No. α β II* β-α  11-Ethyl-3-methylimidazolium acetate ² 143314-17-4 0.57 1.06 0.97   0.49 2 1-Octyl-3-methylimidazolium chloride ¹ 64697-40-1 0.31 0.98 1.03  0.67  3 1-Decyl-3-methylimidazolium chloride ¹ 171058-18-7 0.31 0.980.97   0.67  4 1-Butyl-3-methylimidazolium chloride ¹ 79917-90-1 0.320.95 1.13   0.63  5 1-Butyl-3-methylimidazolium acetate ¹ 284049-75-80.36 0.85 1.06   0.49 1-Butyl-3-methylimidazolium acetate ² 284049-75-80.57 1.18 0.89   0.61  6 1-Hexyl-3-methylimidazolium chloride²171058-17-6 0.48 0.94 1.02   0.46  7 1-Butyl-3-methylimidazolium nitrate¹ 179075-88-8 0.4 0.74 1.04   0.34  8 1-Butyl-3-methylimidazolium174501-65-6 0.52 0.55 0.96   0.03 tetrafluoroborate ¹1-Butyl-3-methylimidazolium 0.77 0.39 1.04   0.38 tetrafluoroborate ²  91-Butyl-3-methylimidazolium 342789-81-5 0.44 0.77 1.02   0.33methanesulfonate ² 1-Butyl-3-methylimidazolium 0.36 0.85 1.04   0.49methanesulfonate ¹ 10 1-Butyl-3-methylimidazolium methyl 401788-98-50.39 0.75 1.05   0.36 sulfate ¹ 1-Butyl-3-methylimidazolium methyl 0.530.66 1.06   0.13 sulfate ² 11 1-Butyl-3-methylimidazolium 174899-66-20.60 0.50 1.00 −0.10 trifluoromethanesulfonate ³ 121-(2-Hydroxyethyl)-3-methylimidazolium 1203809-91-9 0.53 0.90 1.04  0.37 acetate ⁴ 13 1-Butyl-3-methylimidazolium butyrate ² 669772-78-50.51 1.23 0.92   0.72 14 1-Butyl-3-methylimidazolium propionate ²914497-10-2 0.48 1.16 0.94   0.68 15 1-Ethyl-3-methylimidazolium nitrate⁴ 143314-14-1 0.48 0.66 1.13   0.18 ^(X) References: ¹ A. Schade; J.Molec. Liqu., 2014, 192, 137-143 ² R. Wilding; Phys. Chem. Phys., 2011,13, 16831-16840 ³ Electronic Supplementary Material (ESI) for PhysicalChemistry Physics, The Owner Societies 2011 ⁴ Shiguo Zhang, Xiujuan Qi,Xiangyuan Ma, Liujin Lu and Youquan Deng; J. Phys. Chem. B 2010, 114,3912-3920

The ionic liquids No. 1 to No. 6 of Table 2 are suitable for dissolvingmeta-aramid fibers and para-aramid copolymer fibers (see Example 2).They have β values above 0.8 and α values lower below 0.6. Generally,ionic liquids with β values above 0.6, preferably above 0.7, morepreferably above 0.8, may be suitable for the inventive methods. Inparticular, the difference of β value minus α value is high in suitableionic liquids. This difference “β−α” describes the free, availablehydrogen bond acceptor capacity. Preferred ionic liquids have adifference value “β−α”≥0.3, more preferably ≥0.45. The ionic liquids No.7 and No. 8 from Table 2 each have a higher α value, lower β value orlower β−α value. In Example 2, these ionic liquids were not able topartially dissolve aramid fibers. No. 9 to No. 15 from Table 2 were notused in the Examples in this way and are included for comparison. WithNo. 9 it is to be expected that the strikingly high β value fromliterature reference 1 (A. Schade, J. Molec. Liqu., 2014, 192, 137-143)might be a measuring error.

In one aspect, the inventive method is characterized in that the ionicliquid meets at least one of the following two criteria:

i) α value <0.6 and β value >0.8;

ii) difference of β value minus α value ≥0.45,

wherein the α value and β value are Kamlet-Taft solvent parameters.

In one aspect, the method is characterized in that the aramid fibers areselected from the group comprising meta-aramid and para-aramidcopolymers.

It has been shown that meta-aramid and para-aramid copolymer fibers,such as Nomex or Technora, may be dissolved or partially dissolved by agreater variety of suitable ionic liquids, while for para-aramid fiberslike Kevlar, the selection of suitable ionic liquids is more limited.The present disclosure and the examples below provide sufficientinformation and rules for a person skilled in the art to select asuitable ionic liquid for the respective type of aramid fiber.

In a further aspect, the method for welding aramid fibers of the groupcomprising meta-aramid and para-aramid copolymers is characterized inthat the ionic liquid comprises a salt, the cation being selected from aquaternary ammonium, phosphonium, guanidinium, pyridinium, pyrimidinium,pyridazinium, pyrazinium, piperidinium, morpholinium, piperazinium,pyrrolium, pyrrolidinium, oxazolium, thiazolium, triazinium,imidazolium, triazolium, protonated guanidinium, and the anion beingselected from

-   -   halide, selected from the group comprising F⁻, Cl⁻, Br⁻    -   carboxylate of the general formula        [R_(n)—COO]⁻  (Vd)    -   wherein R_(n) represents hydrogen, (C₁₋₁₀)alkyl,        (C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, or        heteroaryl, preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl,        (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5-        to 6-membered heteroaryl,    -   carbonate,    -   alkylcarbonate of the general formula        [R_(s)—OCOO]⁻  (Vf)    -   wherein R_(s) is (C₁₋₄)alkyl, in particular methyl carbonate and        ethyl carbonate    -   hydroxide,    -   alkoxide or aryloxide of the general formula        [R_(m)—O]⁻  (Ve)    -   wherein R_(m) represents (C₁₋₁₀)alkyl, (C₃₋₁₀)cycloalkyl,        (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, or heteroaryl,        preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl,        (C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5- to 6-membered        heteroaryl,    -   phosphate PO₄ ³⁻    -   alkyl or dialkyl phosphate, or alkyl- and dialkyl phosphonate of        the general formulas        [R_(u)—OPO₃]²⁻  (Vi)        [R_(u)O—PO₂—OR_(v)]⁻  (Vj)        [R_(u)—PO₃]²⁻  (Vk) or        [R_(u)—PO₂—OR_(v)]⁻  (Vl),    -   wherein R_(u) and R_(v) independently represent (C₁₋₁₀)alkyl,        (C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, or        heteroaryl, preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl,        (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5-        to 6-membered heteroaryl.

In one aspect, the method is characterized in that the aramid fibers arepara-aramid fibers.

One embodiment is a method for welding aramid fibers, wherein the aramidfibers are para-aramid, characterized in that the ionic liquid comprisesa salt,

the cation being selected from a quaternary ammonium, phosphonium,guanidinium, pyridinium, pyrimidinium, pyridazinium, pyrazinium,piperidinium, morpholinium, piperazinium, pyrrolium, pyrrolidinium,oxazolium, thiazolium, triazinium, imidazolium, triazolium, protonatedguanidinium, and the anion being selected from

-   -   fluoride    -   hydroxide,    -   alkoxide or aryloxide of the general formula        [R_(m)—O]⁻  (Ve)

wherein R_(m) represents (C₁₋₁₀)alkyl, (C₃₋₁₀)cycloalkyl,(C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, or heteroaryl, preferably(C₁₋₈)alkyl, (C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, 5- to6-membered aryl, or 5- to 6-membered heteroaryl.

The ionic liquids that have proven especially useful in the method forwelding para-aramid are, in addition to the quaternary cation,characterized by anions that may be referred to as particularlycoordinating.

According to an exemplary embodiment according to the present invention,[A]⁺ in Formula I represents an ammonium cation [R′₁R₁R₂R₃N]⁺, aphosphonium cation [R′₁R₁R₂R₃P]⁺, a sulfonium cation [R′₁R₁R₂S]⁺, aheterocyclic, e.g. heteroaromatic cation, or a guanidinium cation of theformula

wherein

R₁, R′₁, R₂, R′₂, R₃ and R′₃ are independently alkyl, alkenyl, alkynyl,cycloalkyl, cycloalkenyl, aryl, or heteroaryl, wherein the latter 7residues are independently unsubstituted or substituted by

-   -   one or more halogens and/or 1 to 3 residues selected from        (C₁₋₆)alkyl, aryl, heteroaryl, (C₃₋₇)cycloalkyl, halogen,        OR_(c), SR_(c), NR_(c)R_(d), COR_(c), COOR_(c), CO—NR_(c)R_(d),        wherein R_(c) and R_(d) are independently (C₁₋₆)alkyl,        halo(C₁₋₆)alkyl, cyclopentyl, cyclohexyl, phenyl, tolyl, or        benzyl;

or

two of the residues R₁, R′₁, R₂, R′₂, R₃, R′₃ together with theheteroatom to which they are bound form a saturated or unsaturated ring,which is unsubstituted or substituted, and wherein each carbon chain maybe interrupted by one or more heteroatoms selected from the groupconsisting of O, S, NH, or N(C₁₋₄)alkyl.

In case of guanidinium cations, R₁, R′₁, R₂, R′₂, R₃ und R′₃ may alsoindependently represent hydrogen.

In Formula I, a cationic heteroaryl residue with the meaning of [A]⁺ isselected from a 5- or 6-membered heteroaromatic having at least onenitrogen atom as well as optionally one oxygen or sulfur atom and beingunsubstituted or substituted, in particular selected from the group offormulas

wherein

R and R′ independently represent (C₁₋₂₀)alkyl, (C₃₋₁₂)cycloalkyl,(C₂₋₂₀)alkenyl, (C₃₋₁₂)cycloalkenyl, aryl, or heteroaryl, the latter 6residues each being independently, or being substituted by

-   -   one or more halogen residues and/or 1 to 3 residues selected        from the group of (C₁₋₁₀)alkyl, aryl, heterocyclyl,        (C₃₋₇)cycloalkyl, halogen, OR_(c), SR_(c), NR_(c)R_(d), COR_(c),        COOR_(c), CO—NR_(c)R_(d), wherein    -   R_(c) and R_(d) independently represent (C₁₋₆)alkyl,        halo(C₁₋₆)alkyl, cyclopentyl, cyclohexyl, phenyl, tolyl, or        benzyl,

R₄, R₅, R₆, R₇, R₈ independently represent hydrogen, halogen, nitro,cyano, OR_(c), SR_(c), NR_(c)R_(d), COR_(c), COOR_(c), CO—NR_(c)R_(d),(C₁₋₂₀)alkyl, (C₃₋₁₂)cycloalkyl, (C₂₋₂₀)alkenyl, (C₃₋₁₂)cycloalkenyl,aryl, or heteroaryl, wherein the latter 6 residues are independentlyunsubstituted or substituted by

-   -   one or more halogens and/or 1 to 3 residues selected from the        group of (C₁₋₆)alkyl, aryl, heteroaryl, (C₃₋₇)cycloalkyl,        halogen, OR_(c), SR_(c), NR_(c)R_(d), COR_(c), COOR_(c),        CO—NR_(c)R_(d), wherein R_(c) and R_(d) independently represent        (C₁₋₆)alkyl, halo(C₁₋₆)alkyl, cyclopentyl, cyclohexyl, phenyl,        tolyl, or benzyl,

or

two of the residues R, R₄, R₅, R₆, R₇, R₈, which are adjacent to eachother, together with the atom to which they are bound form a ring, whichmay be unsaturated or aromatic, unsubstituted or substituted, andwherein the carbon chains formed by the respective residues areinterrupted by one or more heteroatoms selected from the group of O, S,N, or N(C₁₋₄)alkyl.

Furthermore, in one compound of Formula I cations of formula [A]⁺ may beprotonated forms of the strong bases 1,5-diazabicyclo[4.3.0]non-5-ene(DBN); 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU);1,4-diazabicyclo[2.2.2]octane (DABCO®);1,8-bis-(dimethyl-amino)naphthaline (Proton Sponge®);N,N,N′,N′-tetramethylethylenediamine (TMEDA);4,5-bis-(dimethylamino)fluorene, or1,8-bis-(hexamethyltriaminophosphazenyl)naphthalene.

Particularly preferred cations [A]⁺ are quaternary ammonium cations[R′₁R₁R₂R₃N]⁺, quaternary phosphonium cations [R′₁R₁R₂R₃P]⁺, orguanidinium cations R₃R′₃N(C═NR₁R′₁)NR₂R′₂, wherein R₁, R′₁, R₂, R′₂, R₃und R′₃ independently represent linear or branched (C₁₋₁₀)alkyl, linearor branched (C₂₋₁₀)alkenyl—in particular vinyl und allyl, cyclohexyl,phenyl, benzyl, or tolyl, and in the case of guanidinium cations alsorepresent hydrogen.

Particularly preferred are

-   -   guanidinium (protonated or quaternary guanidine);        1,1,3,3-tetramethylguanidinium,        1,1,2,3,3-pentamethylguanidinium,        1,1,2,2,3,3-hexamethylguanidinium,    -   diethyldimethylammonium, dipropyldimethylammonium,        dibutyldimethylammonium, dihexyldimethylammonium,        dioctyldimethylammonium, triethylmethylammonium,        tripropylmethylammonium, tributylmethylammonium,        trihexylmethylammonium, trioctylmethylammonium,        trimethylethylammonium, trimethylpropylammonium,        trimethylbutylammonium, trimethylhexylammonium,        trimethyloctylammonium, tetramethylammonium, tetraethylammonium,        tetrapropylammonium, tetrabutylammonium, tetrahexylammonium,        tetraoctylammonium, 2-hydroxyethyltrimethylammonium (cholinium),        2-methoxyethyltrimethylammonium (O-methyl-cholinium),        triallylmethylammonium,    -   tetramethylphosphonium, triethylmethylphosphonium,        tripropylmethylphosphonium, tributylmethylphosphonium,        trihexylmethylphosphonium, trioctylmethylphosphonium,        triisobutylmethylphosphonium, tributylethylphosphonium,        octyltributylphosphonium,    -   N-decyl-N-methylpyrrolidinium, N-octyl-N-methylpyrrolidinium,        N-hexyl-N-methylpyrrolidinium, N-butyl-N-methylpyrrolidinium,        N-propyl-N-methylpyrrolidinium, N-ethyl-N-methylpyrrolidinium,        N,N-dimethylpyrrolidinium, N-allyl-N-methylpyrrolidinium,    -   N-decyl-N-methylmorpholinium, N-octyl-N-methylmorpholinium,        N-hexyl-N-methylmorpholinium N-butyl-N-methylmorpholinium,        N-propyl-N-methylmorpholinium, N-ethyl-N-methylmorpholinium,        N,N-dimethylmorpholinium, N-allyl-N-methylmorpholinium,    -   N-decyl-N-methylpiperidinium, N-octyl-N-methylpiperidinium,        N-hexyl-N-methylpiperidinium N-butyl-N-methylpiperidinium,        N-propyl-N-methylpiperidinium, N-ethyl-N-methylpiperidinium,    -   N,N-dimethylpiperidinium, N-allyl-N-methylpiperidinium,        N-butylpyridinium, N-propylpyridinium, N-ethylpyridinium,        N-methylpyridinium, N-decylpyridinium, N-butylpyrrolium,        N-propylpyrrolium, N-ethylpyrrolium, N-methylpyrrolium.

-   Particularly preferred are also 1,3-dimethylimidazolium,    1,2,3-trimethylimidazolium, 1-ethyl-3-methylimidazolium,    1-vinyl-3-methylimidazolium, 1-vinyl-2,3-dimethylimidazolium,    1-propyl-3-methylimidazolium, 1-isopropyl-3-methylimidazolium,    1-butyl-3-methylimidazolium, 1-allyl-3-methylimidazolium,    1-propyl-2,3-dimethylimidazolium,    1-isopropyl-2,3-dimethylimidazolium,    1-allyl-2,3-dimethylimidazolium, 1-ethyl-2,3-dimethylimidazolium,    1-butyl-2,3-dimethylimidazolium, 1-hexyl-3-methylimidazolium,    1-octyl-3-methylimidazolium, 1-decyl-3-methylimidazolium,    1,3-diethylimidazolium, 1,3-dipropylimidazolium,    1,3-dibutylimidazolium.

Also preferred are the protonated forms of the strong bases1,5-diazabicyclo[4.3.0]non-5-ene (DBN);1,8-diazabicyclo[5.4.0]undec-7-ene (DBU);1,4-diazabicyclo-[2.2.2]-octane (DABCO®).

[B]^(a−) in Formula I is preferably:

-   -   fluoride, chloride, bromide, carbonate, alkyl carbonate, methyl        carbonate; phosphate; hydrogen phosphate; hydroxide, alkoxide,        aryloxide,    -   carboxylate of the general formula        [R_(n)—COO]⁻  (Vd)    -   wherein R_(n) represents hydrogen, (C₁₋₈)alkyl,        (C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, aryl, or        heteroaryl, the latter 6 residues optionally being substituted        by one to two residues selected from the group of (C₁₋₇)alkyl,        aryl, heteroaryl, (C₃₋₇)cycloalkyl, OR_(c), SR_(c), NR_(c)R_(d),        COR_(c), COOR_(c), CO—NR_(c)R_(d), wherein R_(c) and R_(d)        independently represent (C₁₋₇)alkyl, cyclopentyl, cyclohexyl,        phenyl, tolyl, or benzyl,

or

-   -   organic phosphate, or phosphonate of the general formulas        [R_(u)—OPO₃]²⁻  (Vi)        [R_(u)O—PO₂—OR_(v)]⁻  (Vj)        [R_(u)—PO₃]²⁻  (Vk), or        [R_(u)—PO₂—OR_(v)]⁻  (Vl),    -   wherein    -   R_(u) und R_(v) independently represent (C₁₋₈)alkyl,        (C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, aryl, or        heteroaryl, the latter 6 residues being unsubstituted or        substituted by one or two residues selected from the group of        (C₁₋₇)alkyl, aryl, heteroaryl, (C₃₋₇)cycloalkyl, OR_(c), SR_(c),        NR_(c)R_(d), COR_(c), COOR_(c), CO—NR_(c)R_(d), wherein R_(c)        und R_(d) independently represent (C₁₋₇-Alkyl, cyclopentyl,        cyclohexyl, phenyl, tolyl, or benzyl.

As carbon-containing organic, saturated or unsaturated, acyclic orcyclic, aliphatic, aromatic, or araliphatic residues with 1 to 8 carbonatoms, the residue R_(n) at the carboxylate of Formula (Vd), theresidues R_(u) and R_(v) at the organic phosphates of Formulas (Vi) and(Vj), and the organic phosphonates of Formulas (Vk) and (Vl)independently represent preferably

-   -   (C₁₋₈)alkyl and their aryl-, heteroaryl-, cycloalkyl-, halogen-,        hydroxy-, amino-, carboxy-, formyl-, —O—, —CO—, —CO—O—, or        —CO—N-substituted components, such as methyl, ethyl, 1-propyl,        2-propyl, 1-butyl, 2-butyl, 2-methyl-1-propyl (isobutyl),        2-methyl-2-propyl (tert-butyl), 1-pentyl, 2-pentyl, 3-pentyl,        2-methyl-1-butyl, 3-methyl-1-butyl, 2-methyl-2-butyl,        3-methyl-2-butyl, 2,2-dimethyl-1-propyl, 1-hexyl, 2-hexyl,        3-hexyl, 2-methyl-1-pentyl, 3-methyl-1-pentyl,        4-methyl-1-pentyl, 2-methyl-2-pentyl, 3-methyl-2-pentyl,        4-methyl-2-pentyl, 2-methyl-3-pentyl, 3-methyl-3-pentyl,        2,2-dimethyl-1-butyl, 2,3-dimethyl-1-butyl,        3,3-dimethyl-1-butyl, 2-ethyl-1-butyl, 2,3-dimethyl-2-butyl,        3,3-dimethyl-2-butyl, heptyl, octyl, phenylmethyl (benzyl),        2-phenylethyl, cyclopentylmethyl, 2-cyclopentylethyl,        3-cyclopentylpropyl, cyclohexylmethyl, 2-cyclohexylethyl,        methoxy, ethoxy, formyl, or acetyl;    -   (C₃₋₇)cycloalkyl und their aryl-, heteroaryl-, cycloalkyl-,        halogen-, hydroxy-, amino-, carboxy-, formyl-, —O—, —CO—, or        —CO—O-substituted components, such as cyclopentyl,        2-methyl-1-cyclopentyl, 3-methyl-1-cyclopentyl, cyclohexyl,        2-methyl-1-cyclohexyl, 3-methyl-1-cyclohexyl, or        4-methyl-1-cyclohexyl;    -   (C₂₋₈)alkenyl and their aryl-, heteroaryl-, cycloalkyl-,        halogen-, hydroxy-, amino-, carboxy-, formyl-, —O—, —CO—, or        —CO—O-substituted components, such as 2-propenyl, 3-butenyl,        cis-2-butenyl, or trans-2-butenyl;    -   (C₃₋₇)cycloalkenyl and their aryl-, heteroaryl-, cycloalkyl-,        halogen-, hydroxy-, amino-, carboxy-, formyl-, —O—, —CO—, or        —CO—O-substituted components, such as 3-cyclopentenyl,        2-cyclohexenyl, 3-cyclohexenyl, or 2,5-cyclohexadienyl;    -   aryl, or heteroaryl with 2 to 8 carbon atoms and their alkyl-,        aryl-, heteroaryl-, cycloalkyl-, halogen-, hydroxy-, amino-,        carboxy-, formyl-, —O—, —CO—, or —CO—O-substituted components,        such as phenyl, 2-methylphenyl (2-tolyl), 3-methylphenyl        (3-tolyl), 4-methylphenyl, 2-ethylphenyl, 3-ethylphenyl,        4-ethylphenyl, 2,3-dimethylphenyl, 2,4-dimethylphenyl,        2,5-dimethylphenyl, 2,6-dimethylphenyl, 3,4-dimethylphenyl,        3,5-dimethylphenyl, 4-phenylphenyl, 1-naphthyl, 2-naphthyl,        1-pyrrolyl, 2-pyrrolyl, 3-pyrrolyl, 2-pyridinyl, 3-pyridinyl, or        4-pyridinyl.

In one aspect, [B]^(a−) is preferably fluoride, chloride, bromide,methyl carbonate, carbonate, hydroxide, methoxide, ethoxide, phenolate,phosphate, methyl sulfate, or ethyl sulfate.

If the anion [B]^(a−) is a carboxylate of Formula (Vd), the residueR_(n) preferably represents hydrogen, phenyl, p-tolyl, linear, orbranched (C₁₋₆)alkyl, such as methyl, ethyl, 1-propyl, 2-propyl,1-butyl, 2-butyl, 2-methyl-1-propyl (isobutyl), 2-methyl-2-propyl(tert-butyl), 1-pentyl, 2-pentyl, 3-pentyl, 2-methyl-1-butyl,3-methyl-1-butyl, 2-methyl-2-butyl, 3-methyl-2-butyl,2,2-dimethyl-1-propyl, 1-hexyl, 2-hexyl, 3-hexyl, 2-methyl-1-pentyl,3-methyl-1-pentyl, 4-methyl-1-pentyl, 2-methyl-2-pentyl,3-methyl-2-pentyl, 4-methyl-2-pentyl, 2-methyl-3-pentyl,3-methyl-3-pentyl, 2,2-dimethyl-1-butyl, 2,3-dimethyl-1-butyl,3,3-dimethyl-1-butyl, 2-ethyl-1-butyl, 2,3-dimethyl-2-butyl,3,3-dimethyl-2-butyl, heptyl, or octyl.

Particularly preferred carboxylates (Vd) are acetate, methoxyacetate,cyanoacetate, propionate, iso-propionate, acrylate, butanoate,iso-butanoate, methacrylate, valerate, pivalate, caprylate, oxalate,malonate, maleinate, fumarate, succinate, glutarate, pyruvate,phthalate, isophthalate, terephthalates. More preferred carboxylates areformate, acetate, propionate, benzoates.

If the anion [B]^(a−) is an organic phosphate of Formulas (Vi) or (Vj),or an organic phosphonate of Formulas (Vk) or (Vl), R_(u) und R_(v)independently preferably represent methyl, ethyl, n-propyl, iso-propyl,n-butyl, iso-butyl, tert-butyl, phenyl, and p-tolyl. Particularlypreferred organic phosphates (Vj) are phosphates, diethylphosphates,dibutylphosphates, bis(2-ethylhexyl)-phosphates, diphenylphosphates,dibenzylphosphates. Particularly preferred organic phosphonates (Vl) arep-methylphosphonate, p-ethylphosphonate, dimethylphosphonate, anddiethylphosphonate.

In a method according to the present invention, to the described ionicliquids may be added 5-75% by weight of metal salts of the formula[M]_(x) ^(b+)[B]_(y) ^(a−)

wherein a, b, x and y independently represent the numbers 1, 2, 3, or 4and wherein the product of x and b is the same as the product of y anda.

Here, preferred metal cations are Cr⁺², Cr⁺³, Co⁺², Co⁺³, Cu⁺¹, Cu⁺²,Fe⁺², Fe⁺³, Mn⁺², Mn⁺³, Ni⁺², Ni⁺³, Ti⁺², Ti⁺³, Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺,Ca²⁺, Ba², Sr²⁺, Zr⁴⁺, Sn²⁺, Sn⁴⁺, Ag⁺, Zn²⁺, and Al³⁺, particularlypreferred are Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺.

Furthermore, up to 75% by weight of any other, non-coordinating liquid(i.e. not dissolving or partly dissolving aramid) may be added asadditive to the described coordinated ionic liquids.

Here, the required amount of the coordinating ionic liquid for partlydissolving an aramid according to the present invention depends on thechemical structure of the aramid, on its polymerization degree, and onthe type of ionic liquid, and may easily be determined by preliminaryexperiments. Numerous experiments for determining the dissolvingbehavior of aramids are shown in Examples 2 and 3.

It has been shown that dissolving an aramid in a coordinating ionicliquid according to the present invention may be adapted via thetemperature. For dissolution, the ionic liquid is thus preferablyheated, e.g. to temperatures of 50° C. to 150° C., preferably 50° C. to100° C., wherein heating may be conducted conventionally or by microwaveradiation. For partial dissolution, it may be preferred to work at lowertemperatures, for example room temperature, in order to prevent completedissolution. Also, it will be apparent to the person skilled in the artthat the treatment time in step a) of the inventive method for partiallydissolving the aramid has to be selected appropriately. In Example 1,the temperature dependency of the dissolution behavior is clearly shownby the fibers of a meta-aramid (Nomex®) being dissolved in1-ethyl-3-methylimidazolium acetate. While 10% w are not completelysoluble at 80° C., heating to 100° C. provides a clear solution.

Furthermore it has been shown that by adding an anti-solvent to asolution of an aramid in a coordinating ionic liquid according to thepresent invention, fibers of the aramid may be precipitated from thesolution. By adding a suitable anti-solvent, the solvent power may bespecifically reduced so that the aramid is only partially dissolved.

Here, an anti-solvent includes a solvent that leads to flocculation ofthe aramid when added to a solution of the aramid. An anti-solvent ischaracterized in that it may enter into strong interaction forces withthe ionic liquid, so that it competes with the dissolved aramid and maycoagulate it as anti-solvent and subsequently precipitate it or preventscomplete dissolution thereof. A preferred anti-solvent includes, forexample, water, alcohols, such as methanol, ethanol, propanol, butanol,glycol, polyalcohols; amines, such as alkylamines, e.g. 1-propylamine,1-butylamine; aldehydes, ketones, such as alkylketones, e.g. acetone,methyl ethyl ketone; halogenated carbohydrates, e.g. dichloromethane,nitriles, such as acetonitrile, nitrocarbohydrates, such asnitromethane, and organic acids, such as carboxylic acids, e.g. formicacid, acetic acid, propionic acid.

Precipitation by adding an anti-solvent thus also represents an optionfor carrying out step c) of the inventive method, in which step thepartially dissolved area of the aramid is re-coagulated afterconnecting. Here, the ionic liquid is treated with an anti-solvent forprecipitating the aramid, and the obtained welded aramid fibers areoptionally isolated from the mixture.

In one aspect, the method is characterized in that the partiallydissolved area of the aramid is re-coagulated by

-   -   i) precipitating the aramid by adding an anti-solvent, or    -   ii) removing the ionic liquid by heating to a temperature above        the thermal decomposition point of the ionic liquid, but below        the thermal decomposition point of the aramid, wherein the ionic        liquid is removed in the form of gaseous decomposition products;        or below its thermal decomposition point, as long as the        coordinating ionic liquid has a vapor pressure sufficiently high        to allow distilling it off optionally in vacuum, or    -   iii) polymerizing the ionic liquid, or    -   iv) a combination thereof.

In a first variation i), the present invention thus provides a methodfor welding aramid fibers, characterized in that the aramid fiberspartially dissolved in the coordinating ionic liquid are treated with ananti-solvent for precipitating the aramid, and the obtained weldedaramid fibers are optionally isolated from the mixture. Here, therequired amount of the anti-solvent for variation i) may easily bedetermined by preliminary experiments. It was, for example, shown thatthe addition of one to five parts of water to a solution of an aramid ina coordinating ionic liquid is sufficient to precipitate aramid fibersfrom the solution.

Aramids are in general thermally (up to approximately 400° C.) verystable, while coordinating ionic liquids are already instable attemperatures around 100-250° C., so that the ionic liquid may decomposeinto molecular compounds at the given temperatures, which may bevolatilized, optionally accelerated by applying vacuum. Since the ionicliquid is removed in this way, aramid fibers may also be caused tore-coagulate by thermal treatment. In this aspect, we refer to ionicliquids as described in WO 2009/027250 to BASF SE. These compounds arein a chemical equilibrium with neutral, non-ionic compounds. Via theseneutral products, the ionic liquid may be distilled off for purificationor separation of contaminations. In the sense of the present method, theionic liquid may thus be removed, as shown in Example 9, and the aramidmay be caused to coagulate. The class of ionic liquids, that may beremoved in this way, comprises, for example, the ones with a cationselected from the group comprising 1,3-dimethylimidazolium,1,3-diethylimidazolium, 1-ethyl-3-methylimidazolium,1-propyl-3-methylimidazolium, and 1-butyl-3-methylimidazolium, and withan anion selected from chloride, formate, acetate, propionate,dimethylphosphate, diethylphosphate, dibutylphosphate,dimethylphosphonate, and carboxylate (linear or cyclic with one or twocarboxylate groups).

Similarly, the present invention provides a method for producing aramidfibers, characterized in that a solution of an aramid in a coordinatingionic liquid is heated to a temperature above the thermal decompositionpoint of the ionic liquid, but below the thermal decomposition point ofthe aramid, so that the ionic liquid is removed in the form of gaseousdecomposition products and the welded aramid is obtained.

A further alternative to restore the native structure of aramid fibersis to remove the ionic liquid as reagent for partial dissolution viachemical modification. Here, the components of the ionic liquid may beselected so that they may be polymerized. After polymerization, they arenot available anymore as suitable means for partially dissolving thearamid, and the aramid precipitates in a welded form. A person skilledin the art may select ionic liquids that may be removed in this waybased on their functionalities. Suitable for polymerization are, forexample, ionic liquids having a cation and/or anion with unsaturatedcarbohydrate side chains. Polymerizable cations are e.g.1-vinyl-3-methylimidazolium, 1-allyl-3-methylimidazolium,triallylmethylammonium, which also polymerize independently of anions.Examples of polymerizable anions are e.g. acrylates and allyl carbonate.However, a skilled person can also select from further coordinatinganions with double bonds.

Several methods may be combined for coagulation that aim at achievingthe native interactions of the aramids and eliminating the partialdissolution of the fibers.

In a further aspect, the invention relates to welded aramid fiber(s).

A welded aramid fiber according to the present invention may, forexample, be a single aramid fiber that is welded with itself in severalareas. For example, a welded aramid fiber may form a ring in which ringclosure has been achieved by welding the ends of the same fiber. Also,welded aramid fibers may comprise a first and at least one other aramidfiber that are welded to each other in at least one contact area. Weldedaramid fibers are also two-dimensional arrangements consisting ofseveral aramid fibers of a fabric welded to each other. A welded areamay, for example, be microscopically discernible via the course of afiber.

In particular, the invention relates to welded aramid fibers obtainablewith an inventive method.

In a further aspect, the invention provides a shaped article made ofwelded aramid.

An inventive shaped article may have different shapes. Aramid fibers andfoils welded side by side allow the manufacture of two-dimensional,planar aramid shaped articles of any size. Suitably only the peripheralareas of the individual units are partially dissolved, contacted witheach other and connected according to the invention. Due to the welding,these junctions have a stability that is comparable with that within theindividual units, but at least better than when the units are attachedto each other via alternative connections. According to the invention,welded foils may, contrary to calendered fabrics, have a surface ofpractically any shape and size. And they have preferred materialproperties compared to bonded surfaces.

For producing three-dimensional shaped articles, aramid fibers or unitsstacked on top of each other may be welded. By means of templates orsupports for providing the external shape, it is also possible to obtainhollow articles and practically any shape. Three-dimensional aramidshaped articles may, depending on their design and purpose, maintaintheir own shape, i.e. contrary to foils have the necessary stiffness inorder to serve as a component. For high strength, it has provenadvantageous to heat the shaped article following coagulation asafter-treatment. This improves the complete removal of the anti-solventand coagulation. It is also advantageous if the fibers are not onlywelded together at intersections, but along longer parallel aramid fiberareas.

Also, aramid fabrics, i.e. arrangements of aramid fibers, may bemodified according to the inventive method so that individual fibers areconnected to each other or merge. Contrary to aramid fabrics known sofar used for bullet-resistant clothing, this also allows the manufactureof stab-proof materials for protective clothing because the individualaramid fibers do not avoid the stabbing tool anymore so that nopenetration gaps evolve.

In addition, the invention provides a conveyor belt comprising a carcasscharacterized in that the carcass comprises a shaped article made ofwelded aramid.

Conveyor belts are used in conveyor facilities or belt conveyors, which,as used in this invention, also comprise transport belts. Here, thecarcass forms the tensile force transferring element and is usedsynonymously with the term tension member.

The carcass of an inventive conveyor belt is preferably made of at leastone foil-like aramid band or parallel aramid fibers that have beenclosed into a ring by means of a welded contact point. This provides thedesired tensile strength along the entire length of the conveyor belt.Furthermore, the inventive carcass guarantees good impact protection.

In addition to these uses, inventive shaped articles are also suitablefor vehicle construction, e.g. for aviation. In aviation, in particular,welded aramid may be used advantageously because the production of astable connection (contrary to loose aramid fibers) eliminates or atleast reduces the need of a surrounding polymer matrix. This does notonly save material, but also solves the problem of saving weight.

The inventive findings regarding dissolving and partially dissolvingaramids with ionic liquids may also be advantageously transferred toother methods with aramids. Examples comprise

-   -   modifying the surfaces of aramids, e.g. for connecting them to        other materials (chemical coupling agent),    -   the production of composites in which an aramid and another        polymer/biopolymer are simultaneously partially dissolved with a        coordinating ionic liquid and welded to each other by adding an        anti-solvent,    -   the production of a solution of aramids in a coordinating ionic        liquid and its general use as adhesive with high strength as        well as for bonding/welding together aramids or other polymers        and biopolymers that are swellable or soluble in the ionic        liquid, wherein the coagulation of the dissolved aramid may be        achieved by adding suitable liquid- or vapor-phase anti-solvents        or by absorption of air humidity, since coordinating ionic        liquids are in particular highly hygroscopic,    -   the removal of aramids from material surfaces in the sense of        cleaning.

In the following examples all temperatures are in degrees Celsius (°C.). In addition to experiments about the dissolution behavior ofaramids, the examples also show exemplary embodiments of the method forwelding aramid fibers as well as the manufacture of shaped articlesaccording to the invention.

Example 1

Nomex® 1780 dtex fibers were dissolved at 80° C. for 3 h in1-ethyl-3-methylimidazolium acetate with magnetic stirring. The additionof 5% w of Nomex® fibers resulted in a clear, viscous solution. When 10%w Nomex® fibers were added, initially not all fibers dissolvedcompletely, but heating to 100° C. again provided a clear, viscoussolution. At 100° C. further addition of Nomex® fibers increased theconcentration to clearly dissolved 20% w, which resulted in a highlyviscous solution.

To 10 g of 1-ethyl-3-methylimidazolium acetate (CAS 143314-17-4; contentHPLC>98% w, water <1% w), 100 mg of Technora® T-240 220 dtex fibers wereadded and magnetically stirred overnight in a sealed round-bottomedflask at 80° C. The aramid copolymer fibers dissolved completely and ahomogenous mixture was obtained. After cooling and adding approx. 50% wof water as anti-solvent, the fibers were re-precipitated and at leastpartially “wound” around a glass capillary by stirring with thecapillary. The addition of approx. 10% w of water resulted in turbiditywith beginning precipitation. It can be assumed that1-ethyl-3-methylimidazolium acetate with up to 10% w of water is stillable to dissolve Technora® fibers.

Example 2

3 mg each of Technora® T-240 220 dtex fibers (para-aramid copolymer) andNomex® 1780 dtex fibers (meta-aramid) were treated with 1 g each of thefollowing, dry (<1% w water) ionic liquids or salts and stirred forthree hours at 100° C. The solvation behavior of the aramid fibers wasobserved and measured, leading to the results shown in Table 3.

TABLE 3 No. Ionic liquid/salt CAS No. Technora ® Nomex ® 11-Ethyl-3-methylimidazolium acetate 143314-17-4 Compl. diss. Compl.diss. 2 1-Butyl-2,3-dimethylimidazolium 98892-75-2 Compl. diss. Compl.diss. chloride 3 1-Butyl-3-methylimidazolium acetate 284049-75-8 Compl.diss. Compl. diss. 4 1-Butyl-3-methylimidazolium chloride 79917-90-1Compl. diss. Compl. diss. 5 1-Ethyl-3-methylimidazolium 848641-69-0Compl. diss. Compl. diss. diethylphosphate 6 1-Ethyl-3-methylimidazoliumbenzoate 150999-33-0 Compl. diss. Compl. diss. 71-Ethyl-3-methylimidazolium 1059603-87-0 Compl. diss. Compl. diss.dimethylphosphonate 8 1-Hexyl-3-methylimidazolium chloride 171058-17-6Compl. diss. Compl. diss. 9 1-Octyl-3-methylimidazolium chloride64697-40-1 Compl. diss. Compl. diss. 10 Tributylmethylammonium acetate131242-39-2 Compl. diss. Compl. diss. 11 Tributylmethylphosphonium947601-89-0 Compl. diss. Gelled dibutylphosphate 12Tributylmethylphosphonium methyl 120256-45-3 Compl. diss. Gelledcarbonate 13 Methoxyethyltrimethylammonium Compl. diss. Compl. diss.acetate 14 1-Ethyl-3-methylimidazolium octanoate 1154003-55-0 Compl.diss. Compl. diss. 15 1-Ethyl-3-methylimidazolium decanoate 1289051-61-1Compl. diss. Compl. diss. 16 1,1,3,3-Tetramethylguanidinium acetate16836-76-3 Gelled Part. diss. 17 1-Decyl-3-methylimidazolium chloride171058-18-7 Gelled Gelled 18 Trioctylmethylammonium acetate 35675-83-3Gelled part. diss. 19 Tetrabutylammonium chloride 1112-67-0 Compl. diss.Gelled 20 1-Benzyl-3-methylimidazolium chloride 36443-80-8 Gelled Gelled21 Trioctylmethylphosphonium chloride 35675-28-6 Compl. diss. part.diss. 22 Trioctylmethylphosphonium acetate Compl. diss. part. diss. 231-Ethyl-3-methylimidazolium methyl 516474-01-4 Unchanged Gelled sulfate24 Tributylammonium chloride 38466-21-6 Unchanged Unchanged 251-Ethyl-3-methylimidazolium sulfate 14331448-5 Unchanged Unchanged 261-Ethyl-3-methylimidazolium 1159682-38-8 Unchanged Unchangedethylphosphonate 27 1-Butyl-3-methylimidazolium nitrate 179075-88-8Unchanged Unchanged 28 1-Butyl-3-methylimidazolium 359845-21-9 UnchangedUnchanged tetrachloroferrate 29 1-Butyl-3-methylimidazolium 174501-65-6Unchanged Unchanged tetrafluoroborate 30 1-Ethyl-3-methylimidazolium145022-45-3 Unchanged Unchanged methanesulfonate 311-Ethyl-3-methylimidazolium 331717-63-6 Unchanged Unchanged thiocyanate32 1-Ethyl-3-methylimidazolium 145022-44-2 Unchanged Unchangedtrifluormethanesulfonate 33 Dimethyl-2-hydroxyethylammonium 932394-20-2Unchanged Unchanged propionate 34 Triethylammonium methanesulfonate93638-15-4 Unchanged Unchanged 35 1-Ethyl-3-methylimidazolium salicylate945611-28-9 Unchanged Unchanged 36 1-Ethyl-3-methylimidazolium888724-53-6 Unchanged Unchanged octadecanoate 37 Lithiumacetatedehydrate 6108-17-4 Unchanged Unchanged 38 Zinc chloride monohydrate21351-92-8 Unchanged Unchanged 39 N-Methylmorpholine-N-oxide 70187-32-5Unchanged Unchanged monohydrate 40 Choline chloride-urea mixture (molar8069-55-4 Unchanged Unchanged ratio 1:2, Deep Eutectic Solvent) 41Tributylmethylphosphonium phosphate/ Compl. diss. Compl. diss. DMSO 1:1

The aramid fibers that were treated with the ionic liquids No. 1-15dissolved completely and could be re-precipitated by adding water asanti-solvent. The ionic liquids 16 to 19 showed strong swellingactivity, but both investigated fiber types were only gelled orpartially, but not completely, dissolved; the ionic liquids 20 to 22only partially dissolved the surface of the aramid copolymer. Also, withthe ionic liquids 16 to 22 the swelling process could be reversed byadding water. All other ionic liquids (24 to 36) left the fibersunchanged. The comparative examples 37 to 40 represent salt melts ororganic compounds that are known to the person skilled in the art fordissolving other fibers, such as N-methylmorpholine-N-oxide monohydrate.It has been shown that these agents are, other than ionic liquids, notsuitable for dissolving aramid fibers. This is obviously also true forNo. 40, a member of the so-called “deep eutectic solvents;” here, thecholinium cation coordinates with urea and not with the aramid fiber.No. 41 shows that a mixture of an ionic liquid with a co-solvent alsohas good dissolution behavior.

This suggests that the dissolution ability depends on the specificproperties of ionic liquids. In addition, there are also differences inthe dissolution properties among the ionic liquids. Regarding thecation, it should be quaternary. Unsuitable for dissolving areprotonated cations, the basicity of which is too low, e.g. ionic liquidsNo. 24, 33, and 34. Dissolution also depends on the anion—which must bestrongly coordinating. Increased basicity of the anion or no acidicprotons also seems to be advantageous.

With increasing size or side-chain length of the quaternary cations,solvent power of the ionic liquid decreases, which is shown by the factthat the aramids are not completely, but only partially, dissolved orgelled. This “modulated” solvent power may be desired to onlysuperficially weld fibers with themselves or other polymer fiberswithout completely dissolving the entire fiber, or for welding togetherthe interior filaments of a fiber. A comparison of the 1-positions ofdifferently substituted 3-methylimidazoline chloride salts shows that1-butyl (No. 3) und 1-octyl (No. 9) completely dissolve the fibers,while 1-decyl (No. 17) results in gelling of both fiber types, as doesthe bulky and inflexible 1-benzyl residue (No. 20).

Interestingly, 1-ethyl-3-methylimidazolium methyl sulfate (No. 23)—theonly representative of the examined compound with sulfur-containinganions—gelled Nomex® fibers. It seems that alkyl sulfates with shortalkyl chains are probably suitable coordinating anions due theirrelatively high oxygen content. These representatives of ionic liquidsare commercially easily available. Consequently, the ionic liquids withthe anion methyl sulfate, or ethyl sulfate may be preferred ionicliquids for inventive methods.

Example 3

3 mg each of Kevlar® K29 fibers (para-aramid) were treated with 1 g eachof the following ionic liquids and stirred for three hours at 100° C.The solvation behavior of the aramid fibers in different ionic liquidswas observed and measured, leading to the results shown in Table 4.

TABLE 4 No. Ionic liquid/salt CAS No. Kevlar ® 11-Ethyl-3-methylimidazolium 133928-43-5 Gelled fluoride 21-Ethyl-3-methylimidazolium 133928-43-5 Compl. diss. fluoride/DMSO 1:1 3Tetrabutylammonium fluoride 22206-57-1 Undissolved hydrate 4Tetrabutylammonium fluoride 22206-57-1 Alm. compl. hydrate/DMSO 1:1diss. 5 Tetrabutylammonium fluoride/ 429-41-4 Compl. diss. DMSO 1:1 6Tetrabutylammonium hydroxide/ 2052-49-5 Compl. diss. DMSO 1:1 7Dimethylmorpholinium hydroxide/ 69013-77-0 Gelled DMSO 1:1 8Tetrabutylammonium fluoride/N- 429-41-4 Gelled methyl-2-pyrrolidone 1:19 Tetrabutylammonium fluoride/N- 429-41-4 Gelled methyl imidazole 1:1 10Tetrabutylammonium fluoride/ 429-41-4 Gelled pyridine 1:1 11Tetrabutylammonium fluoride/ 429-41-4 Unchanged triethylphosphate 1:1

In all other ionic liquids and salts described in Example 2, Kevlar® K29was completely undissolvable. A comparison between Example 2 and Example3 shows that the para-aramid fiber Kevlar® behaves differently from themeta-aramid fiber Nomex® or the para-aramid copolymer Technora®. It canbe assumed that the hydrogen bonds are particularly strong within theinteraction of two para-aramid polymers, on the one hand because of thestrongly polarized N—H bonds through the para-position, and on the otherhand because of the optimal geometric arrangement. Only those ionicliquids are suitable for dissolving a para-aramid fiber that havestrongly coordinating anions. Gelling was shown with the halide fluorideas anion (No. 1), with dry variations being more suitable than a hydrate(No. 3). As anion of an ionic liquid, hydroxide is also suitable forpartial dissolution (No. 6 and 7). Alkoxides and aryloxides are alsosuspected to be suitable anions. Regarding the cations, at least partialdissolution was achieved for heteroaromatic cations (No. 1 and 2) aswell as quaternary ammonium compounds (No. 3 to 10).

In addition, DMSO as co-solvent has shown to be suitable for completedissolution in order to improve the dissolution properties of thepara-aramid fibers (No. 2, 4, 5, and 6). The same effect as co-solventwas also shown for other organic compounds (No. 8 to 10).

Example 4

100 mg of Technora® T-240 220 dtex fibers were treated with 10 g of dry1-butyl-3-methylimidazolium acetate (BMIM-OAc), sealed and put into adrying chamber for 3 hours at 80° C. with shaking from time to time. Aviscous solution was obtained. Then the aramid solution (IL) was treatedwith various solvents (LM) in a volume ratio of IL:LM and stirred. Theeffects of various solvents on a solution of aramid copolymer fibers in1-butyl-3-methylimidazolium acetate are shown in Table 5.

TABLE 5 No. Solvent ε α β π* IL:LM Observation  1 Dimethyl sulfoxide46.5 0.00 0.76 1.00 1:1-1:10 Clear solution  2 N,N-Dimethylformamide36.7 0.00 0.69 0.88 1:1-1:10 Clear solution  3 N-Methyl-2-pyrrolidone32.2 0.00 0.77 0.92 1:1-1:10 Clear solution  4 N-Methylimidazole — 0.000.82 — 1:1-1:10 Clear solution  5 Pyridine 12.9 0.00 0.64 0.87 1:1-1:10Clear solution  6 Triethyl phosphate 13.0 0.00 0.77 0.72 1:1-1:10 Clearsolution  7 Acetonitrile 35.9 0.19 0.40 0.66 1:5 Spont. floccing  8Acetic acid  6.2 1.12 0.45 0.64 1:5 Spont. floccing  9 Methanol 32.70.98 0.66 0.60 1:5 Spont. floccing 10 Nitromethane 35.9 0.22 0.06 0.751:5 Spont. floccing 11 2-Propanol 19.9 0.76 0.84 0.48 1:5 Spont.floccing 12 Water 78.4 1.17 0.47 1.09 1:5 Spont. floccing 131-Butylamine  5.4 0.00 0.72 0.31 1:5 Slow floccing 14 1-Propylamine — —— — 1:5 Slow flocking 15 Acetone 20.6 0.08 0.48 0.62 1:5 Slow flocking16 Dichloromethane  8.9 0.13 0.10 0.73 1:5 Slow flocking 17 1,4-Dioxane 2.2 0.00 0.37 0.49 1:5 Inmiscible, turbidity 18 Ethyl acetate  6.0 0.000.45 0.45 1:5 Inmiscible, turbidity 19 Tetrahydrofuran  7.6 0.00 0.550.55 1:5 Inmiscible, turbidity 20 Toluol  2.4 0.00 0.11 0.49 1:5Inmiscible

The results from Table 5 show the behavior after addition of thesesolvents and puts this behavior in relation to the relative dissociationconstant ε and the Kamlet-Taft solvent parameters α, β and π* (“Solventsand Solvent Effects in Organic Chemistry”; Christian Reichardt,WILEY-VCH, Weinheim 2003, ISBN 3-527-30618-8; Kamlet, M. J.; Abboud, J.L.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027; Kamlet, M. J.; Taft,R. W. J. Am. Chem. Soc. 1976, 98, 377; Kamlet, M. J.; Hall, T. N.;Boykin, J.; Taft, R. W. J. Org. Chem. 1979, 44, 2599; Taft, R. W.;Kamlet, M. J. J. Am. Chem. Soc. 1976, 98, 2886).

Based on the results in Table 5, the solvents may be divided in threegroups:

Group III:

The solvents No. 17-20 show low dielectric constants (ε<8), lackhydrogen bond donor properties (α=0) und have low to medium hydrogenbond acceptor properties (β<0.55). The interactive forces of thesesolvents are too low to be able to dissolve the coordinating ionicliquid and are thus not suitable. This group comprises e.g. esters,ethers, hydrocarbons.

Group II:

The solvents No. 7-16 show either low dielectric constants of c=5-9, butvery high values for a (acetic acid), 3 (1-butylamine), or π*(dichloromethane), or they show high dielectric constants of c=20-78 andat least a medium to high value for α, β and π*. They are thuscharacterized in that they may enter into a strong interaction with theionic liquid, so that they may compete with the dissolved aramid andcoagulate it as anti-solvent and ultimately precipitate it. Particularlysuitable are such solvents that show a high α value (α>0.7), i.e. actvia hydrogen bonds as strong donors and block the hydrogen bondacceptors—i.e. the coordinating anions—in the ionic liquid. This groupcomprises water, alcohols, carboxylic acids, amines, aldehydes, ketones,dichloromethane, acetonitrile, and nitromethane, particularly preferredwater; alcohols such as methanol, ethanol, propanol, butanol, glycol,polyalcohols; carboxylic acids such as formic acid, acetic acid,propionic acid; ketones such as acetone, methyl ethyl ketone. Ingeneral, solvents are preferred with ε≥5, α≥0.22, and β and/or π*≥0.4,particularly preferred are those with ε≥5, α≥0.7 and β and/or π*≥0.4.

Group I:

The solvents No. 1-6 show medium to high dielectric constants ofc=13-47, lack hydrogen bond donor properties (α=0), have high hydrogenbond acceptor properties (β>0.6) and very high bipolarity/polarizabilityπ*>0.7).

They are obviously able to maintain in solution the Kevlar polymerdissolved in the ionic liquid by coordinative bonds together with theionic liquid; since the high values of ε and β are on the one handtypical for solvents, which—as is known to the skilled person—are goodin dissolving ions, and at the same time no hydrogen bonds to the anionsare formed (α=0), the coordinative bonds of the anions to the Kevlarpolymer are obviously maintained and the polymer also remains insolution. This group thus comprises all high-polar aprotic solvents,such as aromatic nitrogen heterocycles, cyclic and linear carboxylicacid amides, sulfoxides, sulfones, alkyl phosphoric esters and amides,cyclic carbonic acid esters. Preferred are in general those solventsthat have ε≥10, α<0.05, β>0.60 and/or π*>0.65, such as for examplepyridines, dimethyl cyanamides, chinolines, N,N-dimethyl formamides,N,N-dimethyl acetamides, N,N-diethyl acetamides, dimethyl sulfoxides,triethyl phosphates, trimethyl phosphates, tributyl phosphates,N-methylpyrrolidones, dimethyl phthalates, N,N,N′,N′-tetramethyl urea,thiolane-1-oxides, N,N,N′,N′-tetramethyl guanidines, N-alkyl imidazoles,anilines, dialkyl sulfoxides, diaryl sulfoxides.

The results described in Example 4 show that solvents of Group 1 areideal co-solvents to e.g. adjust the viscosity and the solidifying pointof the ionic aramid solution, to modulate the solvation properties ofthe ionic liquid (see Example 3), to conduct chemical reactions andmodifications of the dissolved Kevlar polymer, to dissolve educts forsuch reactions, to dissolve additives, to modify surface tension andwettability, to reduce foams, to modify or partially dissolve materialsurfaces to which the aramid is to be connected or welded (see Example5), etc. They also show that solvents of Group II are suitable asanti-solvents in order to cause the dissolved aramid to re-coagulate andprecipitate (see Examples 1 to 3), or in lower dosages they are suitableto reduce the solvent power of the ionic liquid and adjust it to adesired value. The latter can e.g. be necessary to weld aramid fiberswith themselves or with other polymer fibers only superficially withoutcompletely dissolving the entire fiber, or to weld together the interiorfilaments of a fiber.

Example 5

To 60 mg of Technora® T-240 220 dtex fibers were added 2 g of dry1-butyl-3-methylimidazolium acetate, sealed and put into a dryingchamber for 3 hours at 80° C. with stirring from time to time. Ahigh-viscosity, tenacious solution was obtained. Then the solution wasaspirated into a 1 ml plastic syringe without cannula, provided with afine cannula (“insulin cannula” 0.3×12 mm) and injected into a waterbath. A light yellow thread of the aramid copolymer was obtained, whichcould be taken out of the bath and which showed the typical color of theoriginal fibers after drying.

A solution of 3 mg of Kevlar®-K29 in 1 g of a solvent consisting oftetrabutylammonium hydroxide/DMSO 1:1 was prepared. The solvent wasprepared by mixing a 40% aqueous solution of tetrabutylammoniumhydroxide and dimethyl sulfoxide in a mass ratio of 1:1 (in relation tothe pure tetrabutylammonium hydroxide contained in the 40% aqueoussolution) and removing the water in vacuo at 70° C. und decreasing thepressure to 20 mbar. The aramid was already added at this point anddissolved. After dissolving the aramid, 2 phases were observed: ared-colored, viscous phase containing the dissolved aramid, and acolorless, low-viscosity phase probably consisting of excessive DMSO.The red-colored, viscous phase was aspirated into a syringe. Injectioninto a water bath resulted in the coagulation of the dissolved Kevlar®K29, and a characteristically colored thread was obtained, which couldbe removed after 2 hours waiting time and was thermically after-treatedat 90° C. in the a chamber.

Example 6

4 pieces of approx. 3 cm long und approx. 1 mm thick Technora® T-240 220dtex threads (spun fibers of para-aramid copolymer) were dipped forapprox. 10 seconds into dry 1-butyl-3-methylimidazolium acetate untilthey were complete wetted, and excess ionic liquid was removed by wipingoff with cellulose. The threads prepared accordingly were quickly (theionic liquid is hygroscopic and water acts as anti-solvent) laid down inthe shape of a square with the corners intersecting on an object slideand a second object slide was put on top. The two object slides wereheld together at the edges by means of metal clips so that a constantpressure was applied to the intersecting threads wetted with the ionicliquid.

The experimental setup was put into a drying chamber for one hour at 80°C. and then immersed into a beaker containing water with the clips stillin place. By repeated immersion, leaving for 5 minutes, and removal theionic liquid present between the object slides was washed off thefibers. The clips were removed and a mechanically stable square made ofthe threads was obtained.

Under the microscope it was observed that an untreated thread in theimage consisted of numerous individual, clearly discernible fibers. Incontrast, the individual fibers of the treated fibers were meltedtogether and the intersection point was welded, which could be clearlyseen at the curved transition between the horizontal and the verticalthread.

Example 7

Two approx. 50 cm long pieces of an approx. 2 mm wide Nomex® 1780 dtexyarn (meta-aramid) were each coated by means of a standard 1 cm widebristle brush at one end over a length of 5 cm with the ionic liquid1-ethyl-3-methylimidazolium acetate (EMIM-OAc, CAS 143314-17-4; contentHPLC>98% w, water <1% w) until soaked. As shown in FIG. 1, the ends ofthreads 102 und 103 thus treated were then laid down in an exactlyoverlapping way on a standard object slide 101 made of glass by means ofpincers. The length L of the overlapping area was 5 cm. By means ofcellulose, excessive ionic liquid was removed, taking care not to changethe overlapping position of the two yarn ends.

The object slide thus prepared was then heated for 1 hour at T=90° C. ina drying chamber. The object slide with the yarn pieces was then takenout of the drying chamber, cooled off a little and carefully spayed withdistilled water along the yarn pieces by means of an atomizer, whereinthe meta-aramid gelled in the ionic liquid started coagulating byabsorbing water. The object slide including the yarn pieces was thenrinsed for 2 hours in water at 50° C. (beaker with magnetic stirrer) inorder to remove any remaining ionic liquid EMIM-OAc. The welded yarnpieces were then put into the drying chamber for 2 h at 105° C. forthermal after-treatment.

During heating, the Nomex® yarn became optically translucent, and aftercoagulating with water as anti-solvent a white solid with the same coloras the original Nomex® yarn was obtained. The after-treatment in thedrying chamber resulted in increased hardness of the welded point, ascould be clearly determined haptically.

Example 8

In analogy with Example 7, 5 pieces of welded Nomex® 1780 dtex yarnswere prepared, with three variations a.)-c.) with the followingdifferences:

-   -   a.) Use of undiluted EMIM-OAc, overlapping 5 cm (s. Example 6)    -   b.) Use of an only 20% aqueous solution of EMIM-OAc (application        of small amount), overlapping 5 cm; after 45 min heating to        T=90° C. a second slide was added and pressure was applied to        the two object slides and the Nomex® yarn in between by means of        metal clips, in order to guarantee good mechanical contact        between to two treated yarn ends. It was then heated for further        15 min to T=90° C., followed by rinsing the entire setup for one        hour in warm water at 50° C. (beaker with magnetic stirrer), the        setup was taken apart, and the welded yarns were further treated        as described in Example 6 (rinsing for 2 h at 50° C., thermal        after-treatment 2 h 105° C.)    -   c.) Use of undiluted EMIM-OAc, but overlapping length L only 1        cm.

The tensile strengths of the yarn samples were estimated as follows:

A 20 kg weight consisting of a jerrycan of a suitable size filled withwater was put on a top-loading balance, the balance having a measuringrange of max. 60 kg and an accuracy of d=10 g. The display wascalibrated to 0.00 kg with the weight. The Nomex yarn to be tested wasattached by means of a knot to the handle of the jerrycan, and the otherend was attached to a metal rod (length=20 cm, diameter 1 cm). By slowlylifting the metal rod, load was applied to the yarn until break, thedisplay of the top-loading balance showing a negative value due to theload reduction. The maximum value corresponded to the tensile strength(load at break) in kg, which multiplied with the mean gravitationalacceleration of 9.81 m·s⁻² resulted in the estimated tensile strength inNewton. The following values and standard errors were found:

-   -   Untreated yarn (reference): F_(max)=60.8±2.4 N    -   Example 7a: F_(max)=50.0±4.8 N (82% of reference)    -   Example 7b: F_(max)=47.1±5.9 N (77% of reference)    -   Example 7c: F_(max)=31.4±5.0 N (52% of reference)

It could be shown that in the present, suboptimal basic experiments,already tensile strengths close to the tensile strength of the untreatedNomex® 1780 dtex yarn were achieved.

Example 9

In analogy with Example 7, two approx. 50 cm long pieces of an approx. 2mm wide, spun Nomex® 1780 dtex yarn (meta-aramid) were welded togetherover a length of 5 cm. In contrast to Example 7, however, the aramid wasnot coagulated by adding water after the first thermal treatment of onehour at 90° C., but step c) was carried out by an alternative variation.The ionic liquid EMIM-OAc used was a compound having a chemicalequilibrium with neutral, non-ionic liquids, which may be distilled offas described in WO2009027250 to BASF SE. For removal, the yarn partiallydissolved by means of EMIM-OAc, was subjected to a temperature of 150°C. and a pressure of 0.05 mbar together with the object slide in avacuum drying chamber for 2 hours. By treatment in vacuum, the ionicliquid was removed via neutral, vaporizable molecular compounds and thetwo Nomex® 1780 dtex yarns were welded.

Example 10

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Nomex® 1780dtex yarn (meta-aramid) were covered over a length of 1 cm with theionic liquid 1-hexyl-3-methylimidazolium chloride, CAS 171058-17-6(content HPLC>98% w, water <1% w) and then treated as described forExample 7. A strongly welded yarn was obtained.

Example 11

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Nomex® 1780dtex yarn (meta-aramid) were covered over a length of 1 cm with theionic liquid tributylmethylammonium acetate, CAS 131242-39-2 (contentHPLC>98% w, water <1% w) and then treated as described for Example 7. Astrongly welded yarn was obtained.

Example 12

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Technora®T-240 220 dtex yarn (para-aramid copolymer) were covered over a lengthof 1 cm with the ionic liquid tributylmethylammonium acetate, CAS131242-39-2 (content HPLC>98% w, water <1% w) and then treated asdescribed for Example 7. A strongly welded yarn was obtained.

Example 13

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Technora®T-240 220 dtex, yarn were treated as described for Example 7. A stronglywelded yarn was obtained.

Example 14

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Kevlar® K29yarn (para-aramid) were covered over a length of 1 cm with theformulated ionic liquid 1-ethyl-3-methylimidazolium fluoride (CAS133928-43-5)/DMSO 1:1 (purity ionic liquid HPLC>98% rel. area, water13.1% w) and then treated as described for Example 7. A strongly weldedyarn was obtained.

Example 15

Two pieces of approx. 50 cm long and approx. 2 mm wide, spun Kevlar® K29yarn (para-aramid) were covered over a length of 1 cm with theformulated ionic liquid tetrabutylammonium fluoride hydrate(22206-57-1)/DMSO 1:1 (purity of ionic liquid HPLC>98% rel. area, water5.2% w) and then treated as described for Example 7. A strongly weldedyarn was obtained.

Example 16

An approx. 5 cm×3 cm piece of a fabric (m=0.573 g) made of an approx. 2mm wide, spun Nomex® 1780 dtex yarn (meta-aramid) was coated by means ofa standard 1 cm wide bristle brush with the ionic liquid1-ethyl-3-methylimidazolium acetate (EMIM-OAc, CAS 143314-17-4; contentHPLC>98% w, water <1% w) until soaked. Excessive EMIM-OAc was removed byswabbing with cellulose; renewed weighing determined the amount of theionic liquid thus applied to be m=2.122 g. FIG. 2 shows how the fabric202 thus treated was arranged on a shaping support body 201. The bodywas a glass pipe with a diameter of 15 mm that had been covered with avery thin layer of a commercial silicone oil release agent (Lauda Ultra350). A grid 203 was arranged around the fabric 202. For this, anapprox. 6 cm×6 cm piece of a commercial, mechanically rigid aluminumgrid with a mesh of approx. 2 mm and a thickness of approx. 1 mm wasbent into a cylindrical shape with one open side. The interior wascovered with a very thin layer of silicone oil, then the grid 203 waspushed over the support body 201 with the Nomex® fabric 202. Byattaching clips 204, as indicated in FIG. 2, pressure was applied to themold, so that the cylinder of aluminum grid 203 decreased its diameter,was flushly pressed against the rigid glass pipe 201 and caused thepressure to be transmitted to the EMIM-OAc-treated Nomex® 1780 dtexfabric 202.

The setup thus prepared was then heated in a drying chamber for 2 hoursat T=90° C., which turned the Nomex® fabric into a very viscous,gel-like mass. The mold was then treated for 10 minutes over boilingwater with water vapor, so that the meta-aramid gelled in the ionicliquid started coagulating by absorbing water. The gel-like massreturned back into a solid of the same color as the original Nomex®fabric. The mold was then rinsed for 2 hours in water at 50° C. (beakerwith magnetic stirrer) in order to remove any remaining ionic liquidEMIM-OAc. After removal of the metal clips 204 and of the aluminum grid203, the welded aramid was obtained as an elastic, dimensionally stable,cylindrical hollow body, which showed the negative pattern of thealuminum grid on the outside and was smooth on the inside. This hollowbody was then after-treated for 2 hours at 105° C. in the dryingchamber. A hard, dimensionally stable hollow body of Nomex®1780 dtexwith a mass m=0.550 g was obtained.

Example 17

An approx. 3.5 cm×5 cm piece of a fabric (m=0.414 g) of an approx. 2 mmwide, spun Nomex® 1780 dtex yarn was coated by means of a standard 1 cmwide bristle brush with the ionic liquid 1-ethyl-3-methylimidazoliumacetate (EMIM-OAc, CAS 143314-17-4; content HPLC>98% w, water <1% w)until soaked. Excessive EMIM-OAc was removed by swabbing with cellulose;renewed weighing determined the amount of the ionic liquid thus appliedto be m=2.497 g. For forming a Z-shaped three-dimensional body, a setupas shown in FIG. 3 was prepared. The fabric 302 thus treated was putbetween a first support grid 301 and a second support grid 303. Thesupport grids were two approx. 6 cm×6 cm pieces of a commercial,mechanically rigid aluminum grid with a mesh of approx. 2 mm and athickness of approx. 1 mm, which had been covered with a very thin layerof the commercial silicone oil described in Example 16 as release agent.As shown in FIG. 3, the two aluminum grids 301 and 302 including theEMIM-OAc-treated Nomex®1780 dtex fabric 301 were bent into a Z-shape andpressed together with metal clips 304 so that the pressure wastransmitted to the Nomex® fabric.

The setup thus prepared was then heated in a drying chamber for 2 hoursat T=90° C., which turned the Nomex® fabric into a very viscous,gel-like mass. The mold was then treated for 10 minutes over boilingwater with water vapor, so that the meta-aramid gelled in the ionicliquid started coagulating by absorbing water and the gel-like massreturned back into a white solid of the same color as the originalNomex® fabric. The mold was then rinsed for 2 hours in water at 50° C.(beaker with magnetic stirrer) in order to remove any remaining ionicliquid EMIM-OAc. After removal of the metal clips and of the aluminumgrid, an elastic, dimensionally stable, Z-shaped body was removed, whichshowed the negative pattern of the aluminum grid on the outsides. Thisshaped body was then after-treated for 2 hours at 105° C. in the dryingchamber. A hard, dimensionally stable Z-shaped body of Nomex 1780 dtexwith a mass m=0.397 g was obtained.

The invention claimed is:
 1. A method for welding aramid fiberscomprising: a) treating at least one area of an aramid fiber with anionic liquid so that the aramid is partially dissolved, wherein thearamid fiber is a meta-aramid fiber or a para-aramid copolymer fiber,wherein the ionic liquid meets at least one of the following twocriteria: i) α value <0.6 and β value >0.8; ii) difference of β valueminus α value ≥0.45, wherein the α value and β value are Kamlet-Taftsolvent parameters, b) contacting the aramid fiber via the dissolvedarea with another aramid fiber area, and subsequently c) causing orallowing the partially dissolved area of the aramid to becomere-coagulated, wherein the ionic liquid is not1-ethyl-3-methylimidazolium acetate.
 2. The method according to claim 1,wherein the ionic liquid comprises a salt, wherein the salt cation isselected from a quaternary ammonium, phosphonium, guanidinium,pyridinium, pyrimidinium, pyridazinium, pyrazinium, piperidinium,morpholinium, piperazinium, pyrrolium, pyrrolidinium, oxazolium,thiazolium, triazinium, imidazolium, triazolium, protonated guanidinium,and the salt anion is selected from halide, selected from the groupcomprising F—, Cl—, Br— carboxylate of the general formula[R_(n)—COO]—  (Vd) wherein Rn represents hydrogen, (C₁₋₁₀)alkyl,(C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, orheteroaryl, preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl, (C_(2-_8))alkenyl,(C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5- to 6-memberedheteroaryl, carbonate, alkylcarbonate of the general formula[R_(s)—OCOO]⁻  (Vf) wherein Rs is (C1-4)alkyl, in particular methylcarbonate and ethyl carbonate hydroxide, alkoxide or aryloxide of thegeneral formula[R_(m)—O]⁻  (Ve) wherein R_(m) represents (C₁₋₁₀)alkyl,(C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl, aryl, orheteroaryl, preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl,(C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5- to 6-memberedheteroaryl, phosphate PO₄ ³⁻, alkyl or dialkyl phosphate, or alkyl- anddialkyl phosphonate of the general formulas[R_(u)—OPO₃]²⁻  (Vi)[R_(u) O—PO₂—OR_(v)]⁻  (Vj)[R_(u)—PO₃]²⁻  (Vk) or[R_(u)—PO₂—OR_(v)]—  (VI), wherein R_(u) and R_(v) independentlyrepresent (C₁₋₁₀)alkyl, (C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl,(C₃₋₁₀)cycloalkenyl, aryl, or heteroaryl, preferably (C₁₋₈)alkyl,(C₃₋₈)cycloalkyl, (C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, 5- to 6-memberedaryl, or 5- to 6-membered heteroaryl.
 3. The method according to claim1, wherein the partially dissolved area of the aramid is re-coagulatedby: i) precipitating the aramid by adding an anti-solvent, or ii)removing the ionic liquid by heating to a temperature above the thermaldecomposition point of the ionic liquid, but below the thermaldecomposition point of the aramid, wherein the ionic liquid is removedin the form of gaseous decomposition products; or below its thermaldecomposition point, as long as the coordinating ionic liquid has avapor pressure sufficiently high to allow distilling it off optionallyin vacuum, or iii) polymerizing the ionic liquid, or iv) a combinationthereof.
 4. The method according to claim 1, wherein contacting thearamid fiber via the dissolved area with another aramid fiber area isperformed while applying pressure to the contact area.
 5. The methodaccording to claim 1, further comprising partially dissolving thefurther aramid fiber area with the ionic liquid.
 6. A method for weldingaramid fibers comprising: a) treating at least one area of an aramidpolymer fiber with an ionic liquid so that the aramid polymer ispartially dissolved, wherein the aramid polymer fiber is a para-aramidfiber and not a para-aramid copolymer fiber, wherein the ionic liquidmeets at least one of the following two criteria: i) α value <0.6 and βvalue >0.8; ii) difference of value minus a value >0.45, wherein the αvalue and β value are Kamlet-Taft solvent parameters, and the ionicliquid comprises a salt, wherein the salt cation is selected from aquarternary ammonium, phosphonium, guanidinium, pyridinium,pyrimidinium, pyridazinium, pyrazinium, piperidinium, morpholinium,piperazinium, pyrrolium, pyrrolidinium, oxazolium, thiazolium,triazinium, imidazolium, triazolium, protonated guanidinium, and thesalt anion is selected from fluoride, hydroxide, alkoxide or aryloxideof the general formula [R_(m)—O]⁻. . . (Ve) wherein Rm represents(C₁₋₁₀)alkyl, (C₃₋₁₀)cycloalkyl, (C₂₋₁₀)alkenyl, (C₃₋₁₀)cycloalkenyl,aryl, or heteroaryl, preferably (C₁₋₈)alkyl, (C₃₋₈)cycloalkyl,(C₂₋₈)alkenyl, (C₃₋₈)cycloalkenyl, 5- to 6-membered aryl, or 5- to6-membered heteroaryl, b) contacting the aramid fiber via the dissolvedarea with another aramid fiber area, and subsequently c) causing orallowing the partially dissolved area of the aramid to becomere-coagulated, wherein the ionic liquid is not1-ethyl-3-methylimidazolium acetate.
 7. The method of claim 6, whereinthe partially dissolved area of the aramid is re-coagulated by: i)precipitating the aramid by adding an anti-solvent, or ii) removing theionic liquid by heating to a temperature above the thermal decompositionpoint of the ionic liquid, but below the thermal decomposition point ofthe aramid, wherein the ionic liquid is removed in the form of gaseousdecomposition products; or below its thermal decomposition point, aslong as the coordinating ionic liquid has a vapor pressure sufficientlyhigh to allow distilling it off optionally in vacuum, or iii)polymerizing the ionic liquid, or iv) a combination thereof.
 8. Themethod of claim 6, wherein contacting the aramid fiber via the dissolvedarea with another aramid fiber area is performed while applying pressureto the contact area.
 9. The method of claim 6, further comprisingpartially dissolving the aramid fiber area with the ionic liquid. 10.The method of claim 1, wherein the aramid fiber is a meta-aramid fiber.